This invention relates to a rock drill having a rotary motor and a percussion unit. Such a rock drill is known from U.S. Pat. No. 5,415,240. The rotation gear of said drill is driven by a rotary motor. The percussion unit is controlled by a rotary valve which is driven via a gear link by the rotary motor. This has the advantage that the percussion frequency is proportional to the rotation rate of the rotary motor and that the percussion unit stops automatically in case the drill steel gets jammed. This saves a substantial amount of time and drilling equipment. The rock drill has therefore very much proved itself. Unfortunately it is quite complex in construction.
In EP-A-145 701 a further rock drill is described in which a pilot operated valve is arranged in the supply line to the percussion motor. The supply pressure to the rotary motor acts as a pilot pressure for the valve. When the drill steel gets stuck in the hole, the turning rate of the rotary motor is reduced and therefore the supply pressure to the motor increases so that the valve closes the supply to the percussion motor. This solution requires a predetermined supply pressure to the rotary motor. Therefore, this pressure cannot be adapted to different rock formations. This rock drill therefore is relatively inflexible in its application.